Electrocardiogram (ECG) signals are commonly used to determine the status of the electrical conduction system of the human heart. As practiced, an ECG recording device is commonly attached to the patient via ECG leads connected to skin electrodes arrayed on the patient's body so as to achieve a recording that displays the cardiac waveforms in any one of 12 possible vectors.
To diagnose and measure cardiac events, the cardiologist has several tools from which to choose. Such tools include twelve-lead electrocardiograms, exercise stress electrocardiograms, Holter monitoring, radioisotope imaging, coronary angiography, myocardial biopsy, and blood serum enzyme tests. The twelve-lead electrocardiogram is generally the first device used in determining cardiac status prior to implanting a medical device (e.g., a pacemaker) within a patient. Following implantation, the physician can typically use any of a number of ECGs to check the device's efficacy.
Prescription for implantation and programming of the medical devices are typically based on the analysis of waveforms provided using a PQRST electrocardiogram (ECG) and an electrogram (EGM). The waveforms are usually separated for such analysis into the P-wave and R-wave in systems that are designed to detect the depolarization of the atrium and ventricle respectively. Such systems employ detection of the occurrence of the P-wave and R-wave, analysis of the rate, regularity, and onset of variations in the rate of recurrence of the P-wave and R-wave, the morphology of the P-wave and R-wave and the direction of propagation of the depolarization represented by the P-wave and R-wave in the heart.
Since the creation of the first cardiac pacemaker, implantable medical device (IMD) technology has advanced with development of further sophisticated, programmable cardiac pacemakers and pacemaker-cardioverter-defibrillator (PCD) arrhythmia control devices; such devices being designed to detect arrhythmias and dispense appropriate therapies. Detection, analysis and storage of EGM data within the implanted medical devices are well known in the art. The detection and discrimination between various arrhythmic episodes is of considerable interest in order to trigger delivery of an appropriate therapy to the patient (via the implantable medical device).
Monitoring electrical activity of the human heart for diagnostic and related medical purposes is well known in the art. For example, as mentioned, circuitry has been designed for recording ECG signals from multiple lead inputs. Similarly, other designs have employed multiple electrode systems that combine surface EKG signals for artifact rejection. The primary application of multiple electrode systems in such designs appears to be vector cardiography from ECG signals taken from multiple chest and limb electrodes. This is a technique for monitoring the direction of depolarization of the heart including the amplitude of the cardiac depolarization waves.
Numerous body surface ECG monitoring electrode systems have been implemented to detect the ECG and conduct vector cardiographic studies. For example, a four electrode orthogonal array has been applied to the patient's skin both for convenience and to ensure precise orientation of one electrode with respect to the other. Likewise, a vector cardiography system has been used for employing ECG electrodes on the patient in commonly used locations and a hex axial reference system orthogonal display has been used for displaying ECG signals of voltage versus time generated across sampled bipolar electrode pairs.
As the functional sophistication and complexity of implantable medical device systems have increased over the years, it has become necessary for such systems to include communication means between the implanted device and/or an external device, for example, a programming console, monitoring system, and similar systems. For diagnostic purposes, it is desirable that the implanted device be able to communicate information regarding the device's operational status and the patient's condition to the physician or clinician. State of the art implantable devices are available which can transmit or telemeter a digitized electrical signal to display electrical cardiac activity (e.g., an ECG, EGM, or the like) for storage and/or analysis by an external device. As such, the implanted pacemaker is designed to detect cardiac signals and transform them into a tracing that is the same as or comparable to tracings obtainable via ECG leads attached to surface (skin) electrodes.
In certain designs, a separate passive sensing reference electrode can be mounted on the pacemaker connector block or otherwise insulated from the pacemaker case. The passive electrode is implemented to provide a sensing reference electrode that is not part of the stimulation reference electrode and thus does not carry residual after-potentials at its surface following delivery of a stimulation pulse. In regard to subcutaneously implanted EGM electrodes, one or more reference sensing electrodes can be positioned (e.g., implanted) on the surface of the pacemaker case for use in monitoring ECG signals. In use, the implanted electrodes can provide an enhanced capability of detecting and gathering electrical cardiac signals via an array of relatively closely spaced subcutaneous electrodes (located on the body of an implanted device).
More recently, alternative methods and apparatus have been used for detecting electrical cardiac signals via an array of subcutaneous electrodes located on a shroud circumferentially placed on the perimeter of an implanted pacemaker. Such designs allow direct incorporation of the electrode into a feedthrough. Depending on the design, feedthrough ferrules may be welded individually into desired positions around the perimeter of an implantable pacemaker and then the feedthrough/electrodes are fabricated into the existing ferrules. Alternatively, the complete feedthrough/electrode assembly may be fabricated and then welded as one body into the pacemaker. These feedthrough/electrode assemblies are electrically connected to the circuitry of an implantable pacemaker to create a leadless Subcutaneous Electrode Array (SEA) for the purpose of detecting cardiac depolarization waveforms displayable as electrocardiographic tracings on an external device in communication with the pacemaker. When the programming head of a programmer is positioned above an implanted device equipped with a leadless SEA electrocardiographic tracing waveforms may be displayed and viewed on the programmer screen.
However, the use of such subcutaneous electrodes have revealed certain shortcomings. For example, one shortcoming involves the efficiency and effectiveness of connecting the electrodes to the internal circuitry of the pacemaker. The embodiments of the invention are directed to overcoming, or at least reducing the effects of, this and/or other shortcomings.